The present invention relates generally to an underground boring technique, especially one which is intended to install underground utility cables, and more particularly to a specific technique for using locate point measurements in predicting the depth of a boring tool at a forward point along a particular path of movement of the boring tool.
Installing underground utility cable using a steerable boring tool is well known in the art. Various examples are described in continuing Mercer U.S. Pat. Nos. 5,155,442, 5,337,002 and 5,444,382 and pending U.S. application Ser. No. 442,481, filed May 16, 1995 which is a continuation of Mercer U.S. Pat. No. 5,444,382 (collectively referred to herein as the Mercer Patents), all of which are incorporated herein by reference. An example of the prior art Mercer technique is best illustrated in FIG. 1 herein which corresponds to FIG. 2 in the Mercer Patents. For purposes of clarity, the reference numerals used in the Mercer Patents have been retained herein for like components.
As seen in FIG. 1, an overall boring machine 24 is positioned within a starting pit 22 and includes a length of drill pipe 10, the front end of which is connected to the back end of a steerable boring head or tool 28. As described in the Mercer Patents, the boring tool includes a transmitter or sonde for emitting a dipole magnetic field 12 which radiates in front of, behind and around the boring tool, as illustrated in part in FIG. 2. A first operator 20 positioned at the starting pit 22 is responsible for operating the boring machine 24, that is, he or she causes the machine to let out the drill pipe, causing it to push the boring tool forward. At the same time, operator 20 is responsible for steering the boring tool through the ground. A second locator/monitor operator 26 is responsible for locating boring tool 28 using a locator or receiver 36. The boring tool is shown in FIG. 1 being guided around an obstacle 30 at a generally constant depth beneath a reference surface 32 until it reaches a termination pit 34. The locator/monitor operator 26 holds locator 36 and uses it to locate the surface position directly above tool head 28. Once operator 26 finds this position, the locator 36 is used to determine the depth tool head 28. Using the particular locator of the present invention, operator 26 can also determine the orientation (yaw, pitch and roll) of tool head 28 and other information pertinent to the present invention, as will be described hereinafter. This information is passed on to operator 20 who uses it to steer the boring tool to its target.
As stated above, the overall arrangement illustrated in FIG. 1 may be used to install underground utility cable. After the boring tool reaches termination pit 34, the cable is connected to the drill pipe and pulled into position within the ground as the drill pipe is pulled back through the underground tunnel to starting pit 22. Once the utility cable is so installed, it would be quite desirable to have a record of where it is actually located within the ground, even if the terrain of the ground changes with time. As will be seen hereinafter, the present invention fulfills this desire in an uncomplicated and reliable way using much of the technology described in the Mercer Patents.
As will be described in more detail hereinafter, a method is disclosed as part of an overall process in which a boring tool, for example boring tool 28 is moved through the ground within a given region along a particular path in an orientation which includes pitch. A locating signal is transmitted from the boring tool which signal exhibits a field defined forward point within a reference surface which field defined forward point is vertically above an inground forward point on the particular path through which the boring tool is likely to pass. The method establishes a predicted depth of the boring tool at the inground forward point by first identifying the field defined forward point. The signal strength of the locating signal is then measured at the field defined forward point as being representative of the depth of the boring tool at an inground upstream point which is the current location of the boring tool. With the boring tool at the upstream inground point, the pitch of the boring tool is determined. Using the measured signal strength and the determined pitch, the predicted depth of the boring tool is determined for the inground forward point based on the boring tool moving along an approximately straight path to the inground forward point.